Multiple transmitters generate multiple polarization multiplexing signals at a given frequency interval, and the polarization multiplexing signals are combined by a combiner to generate a wavelength division multiplexing signal, and the wavelength division multiplexing signal is transmitted to a receiver by using an optical fiber system. The receiver receives the wavelength division multiplexing signal by means of polarization multiplexing coherent reception, uses a demultiplexer to demultiplex the wavelength division multiplexing signal to obtain a signal of a current channel, then uses a polarization splitter to split the signal into two signals, X/Y signals, respectively inputs the X/Y signals to two 90-degree mixers, and moves the signal to a baseband by beating with a local laser signal. Electrical signals received by using four photoelectric detectors pass through four analog-to-digital converters to obtain four digital signals, respectively marked as XI, XQ, YI, and YQ. Finally, the four digital signals are processed by a digital signal processing chip of the receiver to restore data.
In the communications field, with improvement of a system baud rate, bandwidth occupied by a signal carrier is becoming larger. Therefore, when bandwidth of hardware of a transmitter and a receiver is insufficient, a signal loss occurs when a signal passes through the transmitter or the receiver, affecting overall system performance. For a wavelength division multiplexing system, signals are generally loaded on multiple single carriers that are at a fixed interval, and when the bandwidth of the single-carriers increases, inter-channel interference increases, degrading system performance. To improve receiving performance of a system, a two-tap finite impulse response (Finite Impulse Response, FIR) filter and a two-state maximum likelihood sequence detection (Maximum Likelihood Sequence Detection, MLSD) receiver proposed in pages 1664 to 1676 in the article Approaching Nyquist Limit in WDM Systems by Low-Complexity Receiver-Side Duobinary Shaping published in 2012 may be used. Alternatively, a receiver proposed in International Patent Application No. PCT/CN2013/084032 entitled “Methods for Sending and Receiving Signals, Corresponding Device and System” published in 2013 may be used, to improve quality of bandwidth-limited signals received.
There are different channel conditions such as different system bandwidth or different optical signal noise ratios (Optical Signal Noise Ratio, OSNR) and different transmission requirements such as different channel interval requirements of a wavelength division multiplexing (Wavelength Division Multiplexing, WDM) system, and impact caused by different channel conditions and different transmission requirements cannot be eliminated when data is restored from signals processed by the digital signal processing chip. Therefore, if a receiving system having a fixed filtering coefficient is used, the system performance of the receiver will be relatively poor due to mismatching.